The invention relates to a sealing system for large-surface thin parts to be interconnected, used for the mutual sealing of adjacent separating plates, in particular of bipolar plates with interposed ion-exchange membrane units (MEA) in fuel cells, using sealing elements made of polymer material, in which the individual sealing elements, inserted into recesses, form an integral unit with the respective associated bipolar plate.
Electrochemical fuel cells have gained importance in recent time. Various types are available, depending on the building or fuel materials used. Thus, for example, the PCT Application WO 93/06627 deals with such a fuel cell in detail. Regardless of the configuration of the fuel cells, the sealing of the individual gas-containing and liquid-containing channels is of great importance. No more precise statements are made about this in the PCT Application indicated. It is merely noted that the periphery of the individual separating plates is provided with a seal.
The European Patent PA 0 604 683 goes further into the sealing problem and suggests a sealing material made of silicon, rubber or extrudable elastomeric material. The sealing material is injected into the grooves of the separating plates before the plates are assembled. However, such a sealing system has the disadvantage that it very quickly loses elasticity during use, and therefore leakages may possibly occur.
It is also generally known to cement separately produced seals individually into the grooves provided in the separating plates. This process is time-consuming, difficult to control, and involves the danger of production errors. The mechanical stress during assembly often leads to deformation and stretching, making the seals useless. This results in a high rejection rate and a high work input.
The bipolar plates of a fuel cell are separated from each other by the so-called MEA (membrane-electrode assembly). If, for example, the seals of the two separating plates or bipolar plates, which are separated from each other by the MEA, are laterally displaced relatively to each other after the assembly, then an uneven distribution of forces and pressure occurs in the bipolar plates and in the seals which greatly stresses the individual components of the fuel cell. This lateral displacement of the seal can scarcely be avoided because of manufacturing tolerances.
In order to avoid the indicated problems, in the European Patent PA 0774 794, the present inventors suggested forming the separating plates, i.e., providing them with an elevation, in such a way that they form the seal at the same time. However, this presupposes that the separating plates are made of a flexible, plastically malleable material.
The object of the invention is to provide a sealing system which allows simple and quick assembly of the fuel cell and also ensures uniform distribution of forces and pressure in the individual components. The set objective is achieved according to the invention in a sealing system of the general type noted above, in that between the side walls of the sealing elements and the adjacent walls of the recesses, provision is made for free spaces or gaps to accommodate the elastic sealing material which spreads during the compression, the sealing elements only partially filling the recesses or gaps and the height of the sealing elements being greater than the depth of the recesses or gaps before the compression. Thus, a system is created in which the elasticity of the sealing material is not neutralized during the compression due to an impeded transverse expansion. The sealing material is able to spread laterally when pressing the plates together, and a reliable seal is attained. The sealing system also easily accommodates the changes occurring in the expansion of the components during the operation of the fuel cell.
Denoted as a recess in this case is any form on a separating or bipolar plate into which the sealing element is introduced. The recess can be in the form of a trough-like depression, or can also be formed as a groove. However, there must be adherence to the condition that the volume of the sealing elements jutting out beyond the recesses is less than the volume of the free spaces or gaps of the recesses existing when the sealing elements are not compressed. Adequate free space must be provided so that the sealing material can spread laterally during the compression. In this context, a free space or gap must be provided for this purpose on at least one side of the sealing element.
The bipolar plates are produced predominantly of graphite, sheet metal or electrically conductive polymers. Particularly in the case of bipolar plates made of graphite, the adherence of the seal to the graphite is problematic. To achieve the greatest possible reliability in this case, when working with bipolar plates having recesses on one side, the groove is produced in the form of an undercut, or the bottoms of the valleys are provided with blind holes, to thus anchor the sealing elements in the plates. At the same time, it is also beneficial if the blind holes are aligned diagonally with respect to the plate plane, or if they have an enlargement at their bottom. In this manner, the injected seal sticks mechanically to the plate.
According to another specific embodiment, the bipolar plates with recesses on one side can have traversing holes that start from the valley bottom and open through into bore holes of a larger diameter applied from the other side of the plate. Because of this, during the injection process, a portion of the sealing material gets through the plate into the bore hole and spreads out in it. The sealing elements are reliably anchored in this way, which can be used in particular when working with fuel-cell plates for which a double-sided sealing system is not necessary.
For bipolar plates that are to be provided with sealing elements on both sides, it is advantageous if, given recesses lying opposite each other, their valley bottoms are provided with through-holes. In this case, the opposite sealing elements are interconnected during the injection process in a manner that they cannot be lost.
For many bipolar plates, the sealing elements are applied to the outer edges of the plates or to the edges of openings in the plates. In this case, in a further refinement of the inventive idea, the bipolar-plate edges can be stepped on one or two sides and in each case be bordered by a sealing element. The relevant edges are embraced by the sealing element, and the seals are provided on both sides of the plate.
The above-described refinements of the sealing elements ensure that they are secured to the bipolar plates in a manner that they cannot become lost. In addition, the sealing elements of the individual plates are in alignment during their assembly, so that very good, uniform distribution of forces and pressure is achieved in the sealing elements. This uniform distribution of forces and pressure can be supported even further by furnishing the sealing elements with a protruding sealing profile on one of their sealing surfaces, the sealing profile coming to rest against the MEA in the region of the flat sealing surface of the adjacent sealing profile or sealing surface. This is particularly beneficial if the relatively soft and thin membrane is held between the sealing surfaces.